The present invention relates to subterranean stimulation operations and, more particularly, to methods of isolating portions of a subterranean formation adjacent to a highly deviated well bore.
To produce hydrocarbons (e.g., oil, gas, etc.) from a subterranean formation, well bores may be drilled that penetrate hydrocarbon-containing portions of the subterranean formation. The portion of the subterranean formation from which hydrocarbons may be produced is commonly referred to as a “production zone.” In some instances, a subterranean formation penetrated by the well bore may have multiple production zones at various locations along the well bore.
Generally, after a well bore has been drilled to a desired depth, completion operations are performed. Such completion operations may include inserting a liner or casing into the well bore and, at times, cementing a casing or liner into place. Once the well bore is completed as desired (lined, cased, open hole, or any other known completion) a stimulation operation may be performed to enhance hydrocarbon production into the well bore. Examples of some common stimulation operations involve hydraulic fracturing, acidizing, fracture acidizing, and hydrajetting. Stimulation operations are intended to increase the flow of hydrocarbons from the subterranean formation surrounding the well bore into the well bore itself so that the hydrocarbons may then be produced up to the wellhead.
There are almost always multiple zones along a well bore from which it is desirable to produce hydrocarbons. Stimulation operations, such as those mentioned above, may be problematic in subterranean formations comprising multiple production zones along the well bore. In particular, problems may result in stimulation operations where the well bore penetrates multiple zones due to the variation of fracture gradients between these zones. Different zones tend to have different fracture gradients. Moreover, in a situation wherein some zone along a wellbore is depleted, it will have a lower fracture gradient, than a less depleted or nondepleted zone. The more a zone is depleted, the lower the fracture gradient. Thus, when a stimulation operation is simultaneously conducted on more than one production zone, the stimulation treatment will tend to follow the path of least resistance and to preferentially enter the most depleted zones. Therefore, the stimulation operation may not achieve desirable results in those production zones having relatively higher fracture gradients. In some well bores, a mechanical isolation device such as a packer and bridge plugs may be used to isolate particular production zones, but such packers and plugs are often problematic due to the existence of open perforations in the well bore and the potential sticking of the devices. Additionally, in horizontal well bores the well bore is usually contained to one production area. It may be desirable to perform numerous stimulation treatments in a number of zones within the same production area along the length of the horizontal well bore.
One method used to combat problems encountered during the stimulation of a subterranean formation having multiple production zones involves placement of a sand plug into the well bore. When successfully placed, sand plugs isolate downstream zones along the well bore. Once a downstream zone has been isolated with a sand plug, other upstream production zones may be stimulated. Thus, sand plugs are placed so as to isolate zones farther from the wellhead (downstream) from zones closer to the wellhead (upstream). Conventional sand plug operations place sand into a well bore and allow it to settle into a portion of the well bore adjacent the zone to be isolated, so that fracturing fluids and other materials that are later placed into the well bore will not reach the isolated zone. That is, by filling a downstream portion of the well bore with a sand plug, the formation upstream of the sand plug may thereafter be stimulated without affecting the downstream, lower zone. Successively using such a technique allows for the formation of a plurality of stimulated zones along a horizontal well bore, each of which can be stimulated independently of the previously stimulated zones.
One known sand plug method is described in SPE 50608. More specifically, SPE 50608 describes the use of coiled tubing to deploy explosive perforating guns to perforate a treatment zone while maintaining well control and sand plug integrity. In the methods described in SPE 50608, a fracturing stage was performed through treatment perforations and then, once fracturing was complete, a sand plug was placed across the treatment perforations. The sand plug was placed by increasing the sand concentration in the treatment fluid while simultaneously reducing pumping rates, thus allowing a bridge to form. The paper describes how increased sand plug integrity could be obtained by performing a squeeze technique. As used herein the term “squeeze technique” refers to a technique wherein a portion of a treatment fluid comprising particulates is alternately pumped and stopped, thus exposing the treatment fluid to differential pressure against a zone of interest in stages over a period from several minutes to several hours. By alternately pumping and stopping, the treatment fluid is introduced to a zone at a pressure higher than necessary for fluid movement and thus the treatment fluid, and particulates therein are forced into the desired zone. One skilled in the art will recognize that a squeeze technique may be repeated as needed until a desired volume of particulates have been pumped, or until no further volume can be placed into the desired zone. The squeeze technique may be used to develop a sand plug that forms an effective hydraulic seal. However, when the well bore to be treated is a highly deviated well bore, traditional sand plugs, even with the implementation of a squeeze technique, are often ineffective at isolating zones along the highly deviated well bore. Often, in highly deviated well bores, a sand plug may fail to fully plug the diameter of the well bore.
As used herein, the term “highly deviated well bore” refers to a well bore that is oriented between 75-degrees and 90-degrees off-vertical (wherein 90-degrees off-vertical corresponds to fully a horizontal well bore). That is, the term “highly deviated well bore” may refer to a portion of a well bore that is anywhere from fully horizontal (90-degrees off-vertical) to 75-degrees off-vertical.
Other traditional methods of isolation are similarly difficult in highly deviated well bores. Mechanical packers, commonly used in cemented well bores, may be unsuitable for highly deviated well bores. Only a relatively small percentage of the highly deviated completions during the past 15 or more years used a cemented liner type completion; many highly deviated well bores are completed using some type of non-cemented liner or a bare open hole completion. Even those wells where a vertical, or not highly deviated, portion of the well bore was cemented tend not to be cemented in the highly deviated portions of the well bore.